1. Field of the Invention
The present device relates to a phase synchronizing circuit used for phase synchronization between a facsimile receiver and a facsimile transmitter.
2. Description of the Prior Art
In the case of facsimile transmission, transmission of a clear and distinct image is difficult unless the phase of the image on the transmitter side and the phase of the image on the receiver side are properly synchronized in a precise manner at the time of reception. Such being the situation, various sorts of synchronizing circuits, including ones of the follow-up type, such as disclosed in U.S. Pat. No. 3,408,547, are employed. In case phase deviation is present between the transmitter side and the receiver side, the latter type of synchronizer reduces the speed of the drum driving motor on the receiver side to thus ensure proper matching between both phases, and, after proper matching between both phases, the frequency of the input signal to the drum driving motor on the receiver side is maintained at a normal frequency to thus start the transmisssion of an image all of the foregoing have been thus far employed in conventional practice. However, a phase synchronizing circuit of the follow-up type as set forth above is subject to a certain limit in terms of precision, and thus another improved synchronizer of the follow up type has been introduced, this being disclosed in U.S. patent application Ser. No. 492,907 filed July 26, 1974 being the inventor of the present invention. The latter synchronizer, in addition to reducing the speed of the drum driving motor on the receiver side until the time of matching between the receiver and a transmitter, applies a direct-current voltage to the drum driving motor for a certain period of time each time a phase signal time is generated when the receiving drum makes a full revolution. The direct-current voltage thus controls the said motor by proper braking, and both the transmitter and receiver phases are synchronized in a precise manner. However, such a phase synchronizing circuit as just set forth above, is still not free from defects in that, if the drum on the transmitter side and the drum on the receiver side have a large speed difference from each other, such as immediately after the receiving drum has just started the revolution thereof, both phases may be temporarily synchronized in some cases whereby a phase synchronization completion signal would be thus generated at the receiver side. Hence, the subsequent input to the receiving drum driving motor would be maintained at the above-mentioned normal frequency to thus render phase synchronization impracticable. Furthermost a long period of time would be required for effectuating proper phase synchronization, due to the correction by braking each time being only slight.